The present invention relates to reduction- and oxidation-stable iron oxide black pigments having high colour strength, the preparation thereof and the use thereof.
Iron oxide black pigments consist of oxides of divalent and trivalent iron having the magnetite structure. They are prepared either by the one- or two-stage precipitation process from iron(II) salt solutions, as described, for example, in DE 2618058 A1, example 1, or by the Laux process by reduction of nitrobenzene with metallic iron (Ullmanns Encyklopädie der technischen Chemie [Ullmann's Encyclopaedia of Industrial Chemistry], 4th Edition, Volume 18, page 603, Verlag Chemie, Weinheim 1979, and DE 518 929 C1). The aqueous treatment of pastes substantially comprising iron and oxygen and obtained from the wastewaters in the areas of washing, filtration, solid/liquid separation and also faulty batches, etc. (designated below as wastewater pastes), which treatment is described in DE 3 518 093 A1, also leads to iron oxide black pigments. The black pigments thus obtained generally have an undesired brownish shade.
The stabilization of pulverulent iron oxide black to oxidation can be effected by a chemical aftertreatment as described in DE 2 625 106 A1. This stabilizes the pigment, but the colour properties tended to be adversely affected and the ecology of the pigment was not positively influenced. A process, as described in DE 3 620 333 A1, for the thermal treatment of black pigments in a rotary kiln under inert conditions was therefore developed. A further improvement of this process, the heating being carried out not under inert conditions but under slightly oxidizing conditions, is described in DE 3 910 783 A1.
DE 3 620 333 A1 describes iron oxide black pigments which are prepared by the nitrobenzene reduction process and are subjected to a temperature treatment at 200 to 800° C. under a nonoxidizing atmosphere. Iron oxide black pigments having high colour strength can be obtained thereby. Furthermore, the colour quality improves in the desired manner by reduction in the brown shade and an increase in the blue tinge. The bluish shade, characterized by a negative low b* value, is of particular interest for users since it gives rise to the desired deep black colour impression. Black pigments having a positive high b* value have an undesired brownish appearance especially in full shade applications. The method for measuring the b* value is stated in the examples. In practice, however, it has been found that the process gives iron oxide black pigments which do not have completely satisfactory stability to oxidation. In other words, the storage of relatively large amounts of pigment at temperatures above 80° C. must be avoided since an oxidative change in the pigment may occur as a result of heat accumulation.
The pigments obtained by the one- or two-stage precipitation process from iron(II) salt solutions (cf. for example the pigments described in DE 2618058 A1, example 1) show the required stability to reduction in the colouring of plastics but are not sufficiently oxidation-stable for safe transport and with regard to handling. EP 096 885 A1 therefore proposed that iron oxide black pigments obtained by the nitrobenzene reduction process should be heated at temperatures of 400 to 800° C., preferably 600 to 700° C., in a weakly oxidizing atmosphere and optionally milled. The oxidizing atmosphere can most advantageously be adjusted with atmospheric oxygen to a level of 0.1 to 3% by volume, preferably 0.3 to 1.0% by volume, of oxygen.
Iron oxide black pigments having the desired colour properties and good stability to oxidation can be obtained from the processes described above.
In the colouring of thermoplastics, conventional iron oxide black pigments exhibit an undesired colour shift. This colour shift, which is characterized by the ΔE* value, is caused by the reducing environment in the plastic melt; the iron oxide black pigments are not sufficiently stable to reduction. The method for measuring the ΔE* value is stated in the examples.
It was therefore an object of the present invention to provide iron oxide black pigments having the required good stability to reduction, which simultaneously have the necessary stability to oxidation for transport and handling and the desired high colour strength and as blue-tinged a shade as possible.